1. Field of the Invention
This invention relates to a method and apparatus for producinq free flowing particles comprised of frozen droplets made from a solution of a biological or organic material.
2. Background Art
Biological or organic materials, such as those occurring in food products and drugs, are frequently not a solid at room temperature, or exhibit characteristics which cause them not to be free flowing. These materials are often difficult to handle or process and are not readily measured and separated into small quantities. Being organic in nature, these materials tend to spoil or degrade on storage, reducing their useful lifetime and creating quality control problems.
One method of altering the physical characteristics of such materials to make them more free flowing, while simultaneously increasing their shelf life or useful storage period is to freeze them. Often times the material to be frozen is a solution of the biological or organic material. This is particularly true when it is desired to control the concentration of such material closely. It is also helpful to place the biological or organic material into solution when its non diluted form is difficult to handle. By placing the material into solution and then freezing droplets of the material, it is possible to produce free-flowing particles of the desired concentration of organic material. The particles can be stored for long periods of time in a frozen condition with little change in freshness or active strength.
Freezing of liquids into droplets has been accomplished previously; however, the methods used have not been adequate for large scale, rapid, continuous production of consistent product of the type desired for the application which led to development of the method and apparatus described herein.
U.S. Pat. No. 4,712,310 to Roy describes a technique for co spray preparation of homogeneous hybrid powders suitable for preparing tablets. The tablets are useful as reagent carriers for diagnostic assays. Two separate solutions are prepared and simultaneously sprayed from hypodermic needles onto the surface of a moving bath of boiling fluorocarbon refrigerant. The solutions are sprayed through separate spray needles in a manner to form hybrid droplets which are immediately frozen. The frozen droplets are collected and lyophilized.
U S. Pat. No. 4,687,672 to Vitrovsky describes the preparation of free flowing food particles. Particles of solid food, or extruded viscous food, or food in a liquid form pumped through apertures is frozen in a bath of cryogenic liquid, and then fractured into particles which are screened to remove fines. Such fractured particles are somewhat irregular in shape and not as free flowing as desired for applications such as those for which the present invention is intended. The process of the present invention does not require a fracturing step and thus provides a more uniformly shaped particle.
U.S. Pat. No. 4,655,047 to Temple et al. discloses chilling or freezing of liquid food substances, in particular liquid egg. The substance to be frozen, in liquid form, is allowed to fall a short drop height onto the surface of liquid nitrogen. The substance may fall from a tube as a continuous stream or may drip from the tube onto the liquid nitrogen surface. The patent also discloses that the technique might be used for freezing liquid soap and liquids containing bacteria cultures.
U S. Pat. No. 4,077,227 to Larson discloses a method of freezing a liquid material such a blood and semen using a technique which inhibits agglomeration of droplets of the material during freezing. The liquid is first separated into successive drops, each of which is subjected to an electric field to induce an electric charge of the same polarity on each drop. The electronic charge on the drops keeps them apart so they freeze as individual droplets.
U.S. Pat. No. 3,932,943 to Briggs et al. describes a process for preparation of a particulate product of a substance containing at least one biologically active component. The substance is placed in a solution or colloidal suspension and is sprayed above a moving bath of fluorocarbon refrigerant. The spraying is conducted from a sufficient height above the refrigerant that the spray can form into droplets of solution prior to contact with the fluorocarbon refrigerant whereby the droplets are frozen. The frozen droplets are subjected to a vacuum at a temperature such that essentially all of the liquid in the droplets sublimes, forming particles from the frozen droplets. Use of fluorocarbon refrigerant is considered critical so the frozen droplets will float on the refrigerant and so the desired heat transfer rate can be achieved. The typical particle size of the frozen droplets is estimated to be in the range of about 180 microns (0.18 mm), since they were collected on an 80 mesh sieve.
U.S. Pat. No. 3,755,530 to Avila et al. describes a method of treatment of waste solutions, wherein a liquid solution of solid wastes in injected into a refrigerant liquid through an orifice disposed at the bottom of a vessel containing the refrigerant liquid. Water is subsequently removed from the frozen solution by the sublimation of ice in a controlled vacuum.
U.S. Pat. No. 3,721,725 to Briggs et al. discloses a method of making a homogeneous solid particulate blend of solid initial ingredients. The method involves spraying a solution of the solid ingredients into boiling dichlorodifluoromethane or other fluorocarbon refrigerant and lyophilizing the resulting frozen droplets.
U.S. Pat. No. 3,551,533 to Monforte et al. discloses a method of freeze drying an atomized liquid solution, e.g. a solution of refractory type materials. The method requires subsequent removal of the solvent used by sublimation from the frozen droplets to leave a product in the form of a uniform fine powder.
Many of the above methods utilize an atomized spray of the liquid to be frozen, which results in the formation of very fine particles, typically in the size range of 50 to 500 microns (275 to 35 mesh USA Standard) and having a broad distribution of particle sizes. Other methods disclose formation of individual globules or droplets and their disposition onto the surface of a cryogenic liquid. Formation of individual droplets which are allowed to drip or which are extruded one by one is a relatively slow process. In one of the methods, for freezing of liquid food substances, a continuous stream of the liquid flows under pressure from a tube into liquid nitrogen and breaks up into droplets upon contact with the nitrogen surface. Droplets formed in this manner generally are not uniform in size.
Once the droplets or globules of material are formed and frozen within the cooling liquid, it is necessary to recover them from the cooling liquid. Most of the above described patents disclose that the frozen globules are more dense than the cooling liquid, and thus the frozen globules settle to the bottom of the tank holding the cooling liquid. The frozen droplets or globules are then transported by a conveyor means out of the tank containing the cooling liquid. In two of the cases, the frozen droplets or globules are said to be less dense than the freezing refrigerant and to float in the freezing refrigerant. In one of these cases, a liquid food substance is discharged from a nozzle into a trough of cryogenic liquid and the material in the trough is passed through a screen to remove the frozen product prior to recirculation of the cryogenic liquid back to the beginning of the trough. However, this latter method does not deal with the possibility of agglomeration of freezing globules or droplets, since the only motion at the surface of the cryogenic liquid is the flow of the cryogenic liquid along the trough toward the screen. In the other case, the refrigerant liquid is removed from the frozen droplets by sublimation, under a vacuum.
Thus, it would be useful to have a method of forming essentially spherical frozen droplets of uniform size and uniform droplet composition which are comprised of biological or organic material, wherein the method permits droplet formation at a rapid rate while still permitting formation of larger sized individual droplets than can be obtained by atomizing techniques. In addition, it would be useful to have an improved method of removing the frozen droplets from a cryogenic liquid used to freeze them, particularly when the frozen droplets are less dense than the cryogenic liquid and tend to float toward the liquid surface where particle agglomeration with unfrozen liquid droplets can take place.